Sources and control of release of plasma catecholamines in the American eel, {\it Anguilla rostrata\/} (Teleostei)

Abstract

The origin of plasma catecholamines (CAs) in the American eel, Anguilla rostrata, was studied by five approaches: (1) determination of CA content of tissues suspected to release dopamine (DA), norepinephrine (NE) and/or epinephrine (E) into the circulation; (2) removal of tissues with high CA concentrations, followed by determination of its impact on stimulated CA release; (3) measurement of local CA titers in selected regions of the cardiovascular system; (4) identification of CA-ergic cells with histofluorescence and immunohistochemical localization of CA biosynthetic enzymes and opioids. High tissue concentrations of CAs were measured in the walls of the posterior cardinal veins (PCVs), from their caudal origin within the opisthonephric kidney (OPK) to their termination in the ductus Cuvieri. Both PCV tissue concentrations and plasma levels were maximal near the heart suggesting that this most rostral portion of the PCV is the adrenal medulla equivalent (AdMEq). Tyrosine hydroxylase (TH)-positive (i.e. CA-ergic) cells were found in numbers proportional to CA tissue concentrations. Two cell types were distinguished by the presence ("E" cell) or absence ("NE" cell) of phenylethanolamine-N-methyl-transferase immunoreactivity. One or both of these cells appears to contain methionine-enkephalin and/or morphine-like immunoreactive material. All TH-positive cells also contained dopamine-$\beta$-hydroxylase immunoreactivity indicating the absence of an exclusively DA-secreting cell. CA release demonstrated within the OPK suggests that these CAs affect renal and/or hepatic functions. Other organs contain CA concentrations high enough to potentially affect the CA level in the systemic blood, if instantly released. However, neither removal of the brain, pituitary, urophysis, corpuscles of Stannius nor AdMEq significantly altered CA-induced CA release. Control of this release was also investigated. The CA-ergic cells of the American eel are strongly innervated. However, removal of the brain combined with extirpation of the anterior spinal cord did not prevent the CA-tropic effects of CAs. This suggests that some humoral stimuli for CA release do not require the presence of "preganglionic" innervation. ^